Horizontal blinds are known which utilize prismatic slats to retro-reflect incoming solar rays (see, for example, U.S. Pat. No. 4,742,813, which is owned by the assignee of the present application, and European Patent Publication No. 0 199 931 Al, which is incorporated herein by reference). In order to effectively use such prismatic slats in connection with vertical Venetian blinds, the blinds must be opened and closed with great precision so that their prismatic slats can be properly positioned with respect to the incoming solar rays. For instance, when the vertical blinds are closed, their prismatic slats must be evenly spaced from each other. Therefore, while it is important that all vertical Venetian blinds be provided with means for spacing their slats with precision, this is especially true in the case of vertical Venetian blinds employing prismatic slats.